I. Field of the Invention
The present application relates to an engine exhaust gas additive storage system.
II. Description of Related Art
Legislation concerning emissions from vehicles and heavy goods vehicles stipulate, among other things, a reduction in the amount of oxides of nitrogen NOx discharged into the atmosphere. One known way to achieve this requirement is to use SCR (selective catalytic reduction) which allows for reduction of the oxides of nitrogen by injecting a reducing agent, generally ammonia, into the exhaust system. This ammonia may originate from the thermolytic decomposition of a solution of an ammonia precursor, the concentration of which may be the eutectic concentration. An ammonia precursor such as this is generally a solution of urea.
Using the SCR method, the high levels of NOx produced in the engine during combustion at optimized efficiency are treated in a catalytic converter as they leave the engine. This treatment requires the use of the reducing agent at a precise concentration and extreme quality. The solution is thus metered accurately and injected into the exhaust gas stream where it is hydrolysed before converting the oxide of nitrogen (NOx) into nitrogen (N2) and water (H2O).
To do this, it is necessary for the vehicles to be equipped with a tank containing a solution of additive (generally urea) and with a device for metering and injecting the desired amount of additive into the exhaust system.
Various systems for storing and feeding reducing additives have been provided in the prior art, and these can be split roughly into two categories: those which withdraw the required amount of additive directly (and which therefore do not recirculate the additive solution) and those which do recirculate the additive solution.
In the 1st case, the metering device (which may be a pump or a metering valve) can be fed with additive simply under gravity, via a line starting in the bottom of the storage tank. U.S. Pat. No. 5,628,186 describes a system such as this.
In the 2nd case, where the outward and return flows are generally controlled using a pressure regulator, the additive is metered under pressure and recourse is generally had to a pump for generating this pressure. U.S. Pat. No. 6,063,350 describes a system such as this.
In order to be able to meter the solution of additive correctly into the exhaust gases, it is known practice to incorporate into the additive tank elements such as a level gauge, a temperature sensor, a quality sensor, a resistive heating element, etc. The aforementioned patent U.S. Pat. No. 6,063,350 proposes to group these various components together in the baseplate of the pump, positioned on the top wall of the tank. This approach makes it easier to incorporate the system into the vehicle (as all the connections are grouped together in the same place) and makes it possible to reduce the number of openings made in the wall of the tank. However, the fact that the baseplate is situated on the top wall of the tank has numerous disadvantages:                there are high points in the feed and return lines, where there is a risk that the gases resulting from a possible decomposition of the additive may accumulate;        it becomes necessary to provide a trap to prevent the pump from running dry and losing its prime;        gauging becomes somewhat inaccurate as a result of deformation of the end wall of the tank over the course of its life;        the components suffer prolonged exposure to additive vapours, even though these vapours are often corrosive (ammonia is present in the case of urea);        some components may become damaged in the event of freezing, because lumps of solid additive floating at the surface may, as they move around, strike the said components.        